U.S. patent number 6,226,246 [Application Number 09/182,179] was granted by the patent office on 2001-05-01 for apparatus and method for measuring characteristics of optical pickup and/or optical disc.
This patent grant is currently assigned to Disk Ware Co. Ltd., Sony Corporation, Sony Precision Engineering Center (Singapore) Pte, Ltd.. Invention is credited to Yukari Hashimoto, Hideo Kato, Shunsuke Kohama, Akihito Nakayama, Kenji Shintani.
United States Patent |
6,226,246 |
Nakayama , et al. |
May 1, 2001 |
Apparatus and method for measuring characteristics of optical
pickup and/or optical disc
Abstract
A device for measuring characteristics of an optical pickup
and/or an optical disc by which characteristics of tracking signals
can be measured accurately in a noise-free manner. The measurement
device 1 has a sample-and-hold circuit 8 and an analog-to-digital
converter 10 for directly sampling an output of an optical pickup 2
as signals A to F in order to store the resulting digital data in a
second memory 11. An arithmetic-logic unit 12d measures tracking
error signals based on the digital data stored in the second memory
11. The arithmetic-logic unit 12d eliminates unstable portions of
the tracking signals generated responsive to rotational
eccentricity of the optical disc to find data.
Inventors: |
Nakayama; Akihito (Singapore,
SG), Shintani; Kenji (Singapore, SG),
Kohama; Shunsuke (Chiba, JP), Hashimoto; Yukari
(Yokohama, JP), Kato; Hideo (Kanagawa,
JP) |
Assignee: |
Sony Precision Engineering Center
(Singapore) Pte, Ltd. (SI)
Disk Ware Co. Ltd. (Tokyo, JP)
Sony Corporation (Tokyo, JP)
|
Family
ID: |
20429777 |
Appl.
No.: |
09/182,179 |
Filed: |
October 29, 1998 |
Foreign Application Priority Data
|
|
|
|
|
Oct 31, 1997 [SG] |
|
|
9703918 |
|
Current U.S.
Class: |
369/53.14;
369/44.32; G9B/7.064; G9B/7.066 |
Current CPC
Class: |
G11B
7/0901 (20130101); G11B 7/0953 (20130101) |
Current International
Class: |
G11B
7/09 (20060101); G11B 7/095 (20060101); G11B
003/90 () |
Field of
Search: |
;369/44.11,44.13,44.25,44.27,44.28,44.29,44.32,44.34,44.35,53.14,53.12,53.13 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
5933397 |
August 1999 |
Yamashita et al. |
6118739 |
September 2000 |
Kishinami et al. |
|
Primary Examiner: Huber; Paul W.
Attorney, Agent or Firm: Frommer Lawrence & Haug, LLP.
Frommer; William S.
Claims
What is claimed is:
1. An apparatus for measuring characteristics of an optical pickup
and/or an optical disc comprising:
focusing servo control means for controlling a focal point position
of a laser light beam illuminated on the optical disc, based on an
output of a photoelectric converting unit of the optical pickup,
for focusing the laser light beam on a recording surface of the
optical disc;
rotational driving means for rotating the optical disc in an
eccentric state; and
characteristics detection means for detecting a signal for
generating tracking error signals from an output of the
photoelectric converting device of the optical pickup, detecting
the vicinity of a transition point of an illuminated position of
the laser light beam caused by eccentricity, removing signal
portions of the vicinity of the transition point from the signal
for generating tracking error signals and for measuring the
characteristic values of the optical pickup and/or the optical disc
based on the above signal freed of the signal of the vicinity of a
transition point.
2. The apparatus for measuring characteristics of an optical pickup
and/or an optical disc as claimed in claim 1 wherein said
characteristics detection means detects the periods between peak
values of waveforms of the tracking signals from said peak values
for detecting signal portions exceeding the pre-set values of the
periods as the vicinity of the transition point.
3. The apparatus for measuring characteristics of an optical pickup
and/or an optical disc as claimed in claim 1 further
comprising:
analog-to-digital converter means for converting an output of the
photoelectric converting unit of the optical pickup into digital
data, and
memory means for storing digital data converted by said
analog-to-digital converter; wherein
said characteristics detection means measures characteristic values
of the optical pickup and/or the optical disc based on the digital
data stored by said memory means.
4. The apparatus for measuring characteristics of an optical pickup
and/or an optical disc as claimed in claim 1 wherein
said rotational driving means rotates an optical disc in an
eccentric state.
5. A method for measuring characteristics of an optical pickup
and/or an optical disc comprising:
rotating the optical disc in an eccentric state;
detecting a signal for generating tracking error signals from an
output of a photoelectric converting unit of the optical
pickup;
removing signal portions of the vicinity of a transition point in
the movement direction of an illuminated point by the laser light
beam caused by eccentricity; and
measuring the characteristic values of the optical pickup and/or
the optical disc based on said signal freed of signal portions in
the vicinity of the transition point.
6. The method for measuring characteristics of an optical pickup or
an optical disc as claimed in claim 5 wherein the period between
peak values of respective waveforms of the tracking signals is
detected from peak values and detecting signal portions the period
of which exceeds a pre-set value as the vicinity of the transition
point.
7. The method for measuring characteristics of an optical pickup or
an optical disc as claimed in claim 5 further comprising:
converting an output of the photoelectric converting unit of the
optical pickup into digital data;
storing the converted digital data; and
measuring characteristic values of the optical pickup and/or the
optical disc based on the stored digital data.
8. The method for measuring characteristics of an optical pickup or
an optical disc as claimed in claim 5 further comprising:
rotating the optical disc in an eccentric state.
Description
FIELD OF THE INVENTION
This invention relates to an apparatus and a method for measuring
characteristics of an optical pickup and/or an optical disc.
DESCRIPTION OF THE RELATED ART
There has so far been known a device for inspecting characteristics
of an optical pickup used in an optical disc drive. The device for
inspecting characteristics of an optical pickup is used for example
in inspection for shipment or reception of an optical pickup in
connection with whether or not the optical pickup satisfies the
prescribed specifications.
FIG. 1 shows a block diagram of a conventional device 100 for
inspecting characteristics of an optical pickup.
The conventional device 100 for inspecting characteristics of an
optical pickup shown in FIG. 1 is designed to inspect an optical
pickup.
The conventional device 100 for inspecting characteristics of an
optical pickup includes a test bench 102 on which the optical disc
is set, a matrix circuit 103 supplied with an output of a
photodetector of the optical pickup 101 for outputting a playback
signal (RF signal) and a servo control circuit 104 for performing
servo control for reproducing an optical disc based on an output of
the matrix circuit 103.
The conventional device 100 for inspecting characteristics of an
optical pickup also includes n measurement circuits 105a to 105n
for measuring various characteristic values of the optical pickup
based on an output of the matrix circuit 103, a multiplexer 106 for
switching between outputs of the measurement circuits 105a to 105n
and a multiplexer 106 for switching between outputs of the
measurement circuits 105a to 105n. The conventional device 100 for
inspecting characteristics of an optical pickup further includes an
analog-to-digital converter 107 for converting an output of one of
the circuits 105a to 105n selected by the multiplexer 106 into
digital data and a computer 108 for statistically processing output
data of the analog-to-digital converter 107 for displaying the
results.
The optical pickup 101 is a subject of inspection by this device
100 for inspecting characteristics of an optical pickup. The
optical pickup 101 is detachably mounted on this device 100 for
inspecting characteristics of an optical pickup. The optical pickup
101 includes a laser diode, a beam splitter, an objective lens and
a photodetector. This optical pickup 101 condenses a laser light
beam outgoing from a laser diode via beam splitter and an objective
lens on the optical disc. The optical pickup also forms an image of
the reflected light from the optical disc on the photodetector. The
photodetector provided on the optical pickup 101 is a photoelectric
converting device and converts the imaged reflected light into
electrical signals.
The optical pickup 101 usually includes plural photodetectors. For
example, the optical pickup 101 includes a four-segment
photodetectors and a pair of photodetectors arranged on both sides
of the four-segment photodetectors for side spot detection. Outputs
of these photodetectors are routed to the matrix circuit 103.
The test bench 102, on which is set the optical disc, runs this
optical disc in rotation for reproducing the disc. The optical disc
set on the test bench 102 is used as reference for this device 100
for inspecting characteristics of the optical pickup.
The matrix circuit 103 is fed with outputs of the photodetectors of
the optical pickup 101 to generate playback (RF) signals, focusing
error (FE) signals and tracking error (TE) signals from the
photodetector outputs.
If the photodetector provided on the optical pickup 101 is made up
of four-segment photodetectors and both side photodetectors for
detecting side spots, the matrix circuit 103 outputs the following
signals: That is, the matrix circuit 103 finds the sum of the
respective outputs of the four-segment detectors to output the
result as RF signal. The matrix circuit 103 also outputs FE signal
by the astigmatic method. Specifically, the matrix circuit 103
computes the sums of the two photodetectors lying across the
four-segment detectors to find the difference between the sums to
output the resulting difference as the FE signal. The matrix
circuit 103 also computes the difference between the side spot
detecting photodetectors to output the resulting difference as TE
signal.
The matrix circuit 103 routes the RF, FE and TE signals, thus
computed, to the servo control circuit 104 and the measurement
circuits 105a to 105n.
The servo control circuit 104 performs servo control during
reproduction of the optical disc based on the RF, FE and TE
signals. Specifically, the servo control circuit 104 performs
focusing servo control, tracking servo control, thread servo
control and tilt servo control.
The measurement circuits 105a to 105n calculate characteristic
values of the optical pickup 101. The measurement circuits 105a to
105n measure respective different characteristic values. Therefore,
the number of the measurement circuits 105a to 105n corresponds to
that of the characteristic values to be measured.
The measurement circuits 105a to 105n perform filtering, peak
detection or frequency/voltage conversion by analog processing in
order to compute the characteristic values. The first measurement
circuit 105a measures the signal level of an S-shaped curve at the
time of capturing a focusing servo loop based on, for example, the
FE signals. The second measurement circuit 105b measures the level
of the TE signals based on the TE signals. The third measurement
circuit 105c measures the level of the RF signal based on the RF
signal. The fourth measurement circuit 105d measures jitter
components of the RF signal based on the RF signal.
The multiplexer 106 switches between outputs of the measurement
circuits 105a to 105n to route an output of the selected
measurement circuit to the analog-to-digital converter 107.
The analog-to-digital converter 107 converts outputs of the
measurement circuits 105a to 105n supplied via multiplexer 106 into
digital data which is routed to the computer 108. The rate of
conversion of the analog-to-digital converter 107 is low because
the outputs of the measurement circuits 105a to 105n are
substantially at the dc level. For example, the rate of conversion
of the analog-to-digital converter 107 is on the order of 1
KHz.
The computer 108 performs statistic processing of digital data
supplied from the analog-to-digital converter 107 to display the
results.
In the conventional device 100 for inspecting characteristics of an
optical pickup, as described above, the characteristic values of
the optical pickup 101 are measured by, for example, n measurement
circuits 105a to 105n, with the number n corresponding to the
number of the characteristic values to be measured. The measured
results are displayed for the user on the computer 108.
The method for measuring the level of the TE signal by this
conventional device 100 for inspecting characteristics of the
optical pickup is explained in detail.
This device 100 for inspecting characteristics of the optical
pickup measures the level of the TE signal using eccentricity of
rotation of the optical disc.
Such eccentricity of rotation of the optical disc occurs when the
center of the optical disc set on the test bench 102 for rotational
driving is offset from the axis of rotation.
For example, if eccentricity of rotation is caused in optical disc
rotation, the optical disc is set on the test bench 102 so that the
center O of the optical disc D differs from the axis of rotation of
the optical disc D, as shown in FIG. 2.
Therefore, the separation x between a position of irradiation Lx on
the optical disc D of the laser light beam L radiated from the
optical pickup and the center O of the optical disc D is varied
with the rotational position of the optical disc D.
Specifically, if the center O of the optical disc D is furthest
from the point of illumination Lx of the laser light beam L, this
distance x becomes a distance x1 corresponding to the distance
between the axis of rotation and the point of illumination of the
laser light beam Lx plus the distance between the axis of rotation
and the center O of the optical disc D. If the optical disc D is
rotated by 1/4 from the rotational position shown in FIG. 2A, the
distance x becomes equal to the distance x2, as shown in FIG. 2A,
if the optical disc D is rotated by 1/4 from the rotational
position shown in FIG. 2B, the center O of the optical disc D
becomes closest to the point of illumination of the laser light
beam Lx, with the distance x becoming equal to a distance x3
corresponding to the distance between the axis of rotation and the
center O of the optical disc D minus the distance between the axis
of rotation and the center O of the optical disc D, as shown in
FIG. 2C. If the optical disc D is rotated by 1/4 from the
rotational position shown in FIG. 2C, the distance x becomes equal
to x4 as shown in FIG. 2D.
Therefore, if the rotation of the optical disc D undergoes
eccentricity, the point of illumination Lx on the optical disc D of
the laser light beam L illuminated on the optical disc D is varied
in synchronism with the rotational period of the optical disc D.
Specifically, the point of illumination Lx on the optical disc D of
the laser light beam L illuminated on the optical disc D is moved
back and forth through the distances x3 to x1 from the center O of
the optical disc D, as shown in FIG. 3. Thus, the point of
illumination Lx traverses plural recording tracks due to
eccentricity of rotation of the optical disc D.
For measuring the TE signal level by the conventional device 100
for inspecting characteristics of an optical pickup, focusing servo
control is performed on the optical disc subjected to eccentricity
of rotation as described above. With this device 100 for inspecting
characteristics of an optical pickup, the radial position of the
optical pickup with respect to the optical disc is fixed, with the
tracking servo circuit being turned off. With the device 100 for
inspecting characteristics of an optical pickup, the optical disc
is run in rotation, with the focusing servo on and with the
tracking servo off, for detecting the TE signal produced with the
rotational eccentricity using, for example, the measurement circuit
105b.
With the device 100 for inspecting characteristics of an optical
pickup, the TE signal detected by the measurement circuit 105b is
integrated for averaging the TE signal level for measuring the
resulting averaged signal level. The value measured by the
measurement circuit 105b is taken into the computer 108 for display
for the user.
With the device 100 for inspecting characteristics of an optical
pickup, the TE signal level can be measured using rotational
eccentricity of the optical disc, as described above.
Meanwhile, if the TE signal is detected using rotational
eccentricity of the optical disc, the period of the TE signal is
varied responsive to the track traversing velocity of the laser
light illuminated from the optical pickup 10. In particular, at the
turning points of the reciprocal movement of the illuminated
position of the laser light beam, the traversing velocity becomes
zero, so that the TE signal period becomes extremely unstable.
Specifically, the turning points of the reciprocal movement mean a
point at which the illuminated position of the laser light beam on
the optical disc becomes furthest from the center of the optical
disc (FIG. 2A) and a point at which the illuminated position of the
laser light beam on the optical disc becomes closest to the center
of the disc (FIG. 2C).
With the conventional device 100 for inspecting characteristics of
an optical pickup, if the illuminated position of the laser light
beam on the optical disc has turning points, there is produced in
the TE signal a waveform shown at time t1 to t2 of FIG. 4. In the
conventional device 100 for inspecting characteristics of an
optical pickup, this signal portion becomes noise components when
measuring the TE signal, thus disabling correct measurement.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an apparatus
for measuring characteristics of an optical pickup and/or an
optical disc comprising:
focusing servo control means for controlling a focal point position
of a laser light beam illuminated on the optical disc, based on an
output of a photoelectric converting unit of the optical pickup,
for focusing the laser light beam on a recording surface of the
optical disc;
rotational driving means for rotating the optical disc in an
eccentric state; and
characteristics detection means for detecting a signal for
generating tracking error signals from an output of the
photoelectric converting device of the optical pickup, detecting
the vicinity of a transition point of an illuminated position of
the laser light beam caused by eccentricity, removing signal
portions of the vicinity of the transition point from the signal
for generating tracking error signals and for measuring the
characteristic values of the optical pickup and/or the optical disc
based on the above signal freed of the signal of the vicinity of a
transition point.
The characteristics detection means may detect the periods between
peak values of waveforms of the tracking signals from the peak
values for detecting signal portions exceeding the pre-set values
of the periods as the vicinity of the transition point.
The apparatus may further comprise:
analog-to-digital converter means for converting an output of the
photoelectric converting unit of the optical pickup into digital
data, and
memory means for storing digital data converted by the
analog-to-digital converter; wherein
the characteristics detection means measures characteristic values
of the optical pickup and/or the optical disc based on the digital
data stored by the memory means.
The rotational driving means may rotate an optical disc in an
eccentric state.
According to another aspect of the present invention, there is
provided a method for measuring characteristics of an optical
pickup and/or an optical disc comprising:
rotating the optical disc in an eccentric state;
detecting a signal for generating tracking error signals from an
output of a photoelectric converting unit of the optical
pickup;
removing signal portions of the vicinity of a transition point in
the movement direction of an illuminated point by the laser light
beam caused by eccentricity; and
measuring the characteristic values of the optical pickup and/or
the optical disc based on the signal freed of signal portions in
the vicinity of the transition point.
The period between peak values of respective waveforms of the
tracking signals may be detected from peak values and detecting
signal portions the period of which exceeds a pre-set value as the
vicinity of the transition point.
The method for measuring characteristics of an optical pickup or an
optical disc may further comprise:
converting an output of the photoelectric converting unit of the
optical pickup into digital data;
storing the converted digital data; and
measuring characteristic values of the optical pickup and/or the
optical disc based on the stored digital data.
The method for measuring characteristics of an optical pickup or an
optical disc may further comprise rotating the optical disc in an
eccentric state.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention might be more fully understood,
embodiments of the invention will be described, by way of example
only, with reference to the accompanying drawings:
FIG. 1 is a block diagram of a known device for measuring
characteristics of a conventional optical pickup.
FIGS. 2A, 2B, 2C and 2D illustrate rotational eccentricity of a
known optical disc.
FIG. 3 illustrates illuminated points of the laser light on a known
optical disc moved back and forth due to rotational eccentricity of
an optical disc.
FIG. 4 illustrates tracking error signals detected using rotational
eccentricity of a known optical disc.
FIG. 5 is a block diagram of a device for inspecting
characteristics of an optical pickup embodying the present
invention.
FIG. 6 illustrates a photodetector provided on an optical pickup
inspected by the device for inspecting characteristics of an
optical pickup shown in FIG. 5.
FIGS. 7A, 7B and 7C illustrate measured results by the device for
inspecting characteristics of an optical pickup shown in FIG.
5.
DESCRIPTION OF EMBODIMENTS
Referring to the drawings, a preferred embodiment of the present
invention will be explained in detail.
The device for inspecting characteristics of an optical pickup
embodying the present invention, referred to hereinafter as a
device for inspecting characteristics or characteristics inspecting
device, inspects characteristics of an optical pickup used in an
optical disc drive. This sort of the characteristics inspecting
device is used for inspecting specifications or characteristics of
the optical pickup in, for example, shipment or acceptance tests of
the optical pickup.
FIG. 5 shows a block diagram of a device for inspecting
characteristics 1 embodying the present invention.
The device for inspecting characteristics 1 is used for inspecting
characteristics of an optical pickup 2.
The device for inspecting characteristics 1 includes a test bench 3
on which to set an optical disc, a matrix circuit 4 fed with an
output of a photodetector of the optical pickup 2 for outputting a
playback (RF) signal and a servo control circuit 5 for performing
servo control for reproducing an optical disc based on an output of
the matrix circuit 4.
The device for inspecting characteristics 1 also includes a first
analog-to-digital converter 6 for converting RF signals of the
matrix circuit 4 to digital signals and a first memory 7 for
transiently storing output data of the first analog-to-digital
converter 6.
The device for inspecting characteristics 1 further includes first
to sixth sample-and-hold circuits 8a to 8f for sample-holding
outputs of the photodetectors of the optical pickup 2, a
multiplexer 9 for switching outputs of the first to sixth
sample-and-hold circuits 8a to 8f, a second analog-to-digital
converter 10 for converting the outputs of the first to sixth
sample-and-hold circuits 8a to 8f into digital data and a second
memory 11 for transiently storing output data of the second
analog-to-digital converter 10.
The device for inspecting characteristics 1 additionally includes a
computer 12 for computing characteristic values of the optical
pickup 2 based on the digital data transiently stored in the first
memory 7 and in the second memory 11 for displaying the computed
results and for controlling the servo control circuit 5 based on
the computed results.
The optical pickup 2 is the subject of inspection by this device
for inspecting characteristics 1. The optical pickup 2 is
detachably mounted on this device for inspecting characteristics 1.
The optical pickup 2 includes a laser diode, a beam splitter, an
objective lens and a photodetector. This optical pickup 2 condenses
a laser light beam outgoing from a laser diode via beam splitter
and an objective lens on the optical disc. The optical pickup also
forms an image of the reflected light from the optical disc on the
photodetector. The photodetector provided on the optical pickup 101
is a photoelectric converting device and converts the imaged
reflected light into electrical signals.
The optical pickup 2 includes plural photodetectors. FIG. 6 shows
an example of plural photodetectors provided on the optical pickup
6.
The optical pickup 2 includes four photodetectors A to D, arrayed
in a 22 matrix configuration, and photodetectors E and F arrayed on
both sides of the photodetectors A to D for side spot detection, as
shown for example n FIG. 6. These photodetectors A to F are used
in, for example, in a so-called three-spot optical pickup adapted
for radiating three laser light beams to the optical disc. The
photodetectors A to D are irradiated with a main beam as a center
beam in the three-spot system. That is, the photodetectors A to D
are irradiated with reflected light from recording pits recorded on
the recording track of the optical disc. The photodetectors E and F
are arranged on both sides of the photodetectors A to D in the
radial direction of the optical disc. The photodetectors E and F
are irradiated with side beams of the three-spot system. For
example, the photodetectors E and F are irradiated with light
reflected from, for example, the edges of the optical disc
track.
The photodetectors A to F convert the light volume of the
illuminated reflected light into signals A to F. The optical pickup
2 routes these signals A to F to the matrix circuit 4. The optical
pickup 2 sends the signals A to F to the first to sixth
sample-and-hold circuits 8a to 8f, respectively.
The test bench 3, on which is set the optical disc, runs the
optical disc in rotation for reproducing the disc. The optical
disc, set on this test bench 3, is used as a reference for this
characteristics inspection device 1. That is, the characteristics
inspection device 1 measures characteristics of the optical pickup
2 based on the playback signals of the optical disc used as the
reference.
The matrix circuit 4 is fed with signals A to F outputted by the
photodetectors A to F of the optical pickup 2 for generating
playback (RF) signals, focusing error (FE) signals and tracking
error (TE) signals based on these signals A to F. The matrix
circuit 4 generates these RF, FE and TE signals based n the signals
A to F as follows: That is, the matrix circuit 4 computes A+B+C+D
based on the signals A to D to generate RF signals. On the other
hand, the matrix circuit 4 outputs FE signals by the astigmatic
method. Specifically, the matrix circuit 4 computes (A+C)-(B+D)
based on the signals A to D to output the computed results as FE
signals. On the other hand, the matrix circuit 4 computes E-F based
on the signals E and F and sends the computed results as TE
signals.
The matrix circuit 4 sends the computed RF, FE and TE signals to
the servo control circuit 5. Also, the matrix circuit 4 sends the
RF signals to the first analog-to-digital converter 6.
The servo control circuit 5 performs servo control for reproducing
the optical disc based on the RF, FE and TE signals. Specifically,
the servo control circuit 5 drives a biaxial actuator actuating the
objective lens of the optical pickup 2, based on the FE signals, so
that the FE signals will be zero, for performing focusing servo
control. Also, the servo control circuit 5 drives the biaxial
actuator actuating the objective lens of the optical pickup 2,
based on the TE signals, so that the TE signals will be zero, for
performing tracking servo control. The servo control circuit 5
detects dc components of the FE signals to perform thread servo
control of the optical pickup 2 so that this dc component will be
zero. The servo control circuit 5 also performs tilt servo control
for controlling the tilt of the optical disc based on the RF
signals. Meanwhile, this servo control circuit 5 may be provided
with a separate disc tilt detection unit for performing the tilt
servo control.
The first analog-to-digital converter 6 converts the RF signals
supplied from the matrix circuit 4 into digital data at a high
sampling frequency, such as at a sampling frequency of, for
example, 30 MHz. The first analog-to-digital converter 6 sends the
RF signals converted into digital data to the first memory 7.
The first memory 7 transiently stores the RF signals converted into
digital data by the first analog-to-digital converter 6.
The sample-and-hold circuits 8a to 8f are fed from the optical
pickup 2 with the signals A to F as photodetector output signals.
The sample-and-hold circuits 8a to 8f sample-hold the signals A to
F simultaneously using the same clocks. The clocks supplied to
these sample-and-hold circuits 8a to 8f are at frequencies not
lower than, for example, 50 kHz. Thus, the sample-and-hold circuits
8a to 8f repeat the sampling and holding operations with the clock
of not less than 50 kHz as one cycle.
The multiplexer 9 switch between outputs of the sample-and-hold
circuits 8a to 8f for supplying one of the sample-held outputs to
the second analog-to-digital converter 10. This multiplexer 9
operates at a rate not less than six times as high as 50 kHz, if
the sample-and-hold circuits 8a to 8f perform sample-holding
operations with the clocks of 50 kHz, so that the sample-held
outputs of the sample-and-hold circuits 8a to 8f can be supplied
within one clock to the second analog-to-digital converter 10.
The second analog-to-digital converter 10 converts all sample-held
outputs of the sample-and-hold circuits 8a to 8f supplied thereto
via multiplexer 9 into digital data which is supplied to the second
memory 11. This second analog-to-digital converter 10 has a
conversion rate sufficient to convert outputs of the
sample-and-hold circuits 8a to 8f within one cycle of the clocks
supplied to the sample-and-hold circuits 8a to 8f. Since there are
six sample-and-hold circuits 8a to 8f, the second analog-to-digital
converter 10 achieves the conversion at a conversion rate not less
than 300 kHz if the sample-and-hold circuits 8a to 8f repeat the
sample-hold operations by 50 kHz clocks.
The sample-and-hold circuits 8a to 8f, multiplexer 9 and the second
analog-to-digital converter 10 convert the signals A to F outputted
by the photodetectors of the optical pickup 2 independently into
digital data. Moreover, the sample-and-hold circuits 8a to 8f,
multiplexer 9 and the second analog-to-digital converter 10 convert
the signals A to F into digital data at the sampling frequency of,
for example, not less than 50 kHz.
In the characteristics inspection device 1, the signals A to F as
photodetector output signals of the optical pickup 2 can be
converted into digital data by means other than the above-described
sample-and-hold circuits 8a to 8f, multiplexer 9 and the second
analog-to-digital converter 10. For example, the characteristics
inspection device 1 may be comprised of six parallel rows of the
analog-to-digital converters each having the sampling frequency of
50 kHz.
The second memory 11 transiently stores the signals A to F of the
optical pickup 2 converted into digital data by the second
analog-to-digital converter 10.
The computer 12 includes, for example, an interfacing section 12a,
a data storage section 12b, an output section 12c and an
arithmetic-logic unit 12d. The interfacing section 12a outputs a
control signal controlling the servo control circuit to this servo
control circuit 5. The data storage section 12b has stored therein
processing programs corresponding to measurement items of the
optical pickup 2 by the characteristics inspection device 1. The
output section 12c displays measured results of the characteristics
of the optical pickup 2.
The arithmetic-logic unit 12 of the computer 12 reads out the RF
signals converted into the digital data from the first memory 7 for
detecting jitter components of the RF signals based on the read-out
data.
The arithmetic-logic unit 12d of the computer 12 also reads out the
signals A to F, converted into digital data, from the second memory
11, and executes arithmetic-logic operations on the measurement
items for measuring the characteristics of the optical pickup
2.
In carrying out the processing for the respective measurement
items, the arithmetic-logic unit 12d of the computer 12 performs
the following arithmeticlogic operations on the data stored in the
first memory 7 and the second memory 11. For example, the
arithmetic-logic unit 12d performs filtering, peak level
calculations, calculations of the waveform period, calculations of
the phase difference of two signals, signal extraction by a level
window, signal extraction by a periodic window and calculations of
the ac and dc signal components.
The items of measurement by the characteristics inspection device 1
are hereinafter explained.
The present characteristics inspection device 1 measures the
following items for searching the characteristics of the optical
pickup 2:
RF signal level (P1)
I.sub.TOP and I.sub.BOTTOM of the RF signal (P2)
Jitter of RF signal (P3)
Beam position of the main beam (P4)
TE signal level (P5)
E-F balance (P6)
E-F phase difference (P7)
S-letter level (P8)
S-letter balance (P9)
Defocusing (P10)
Cross-talk (P11)
Astigmatic Aberration (P12)
The processing program for these items of measurement are stored in
the data storage section 12b as processing programs P1 to P12. The
arithmetic-logic unit 12d reads out from the data storage unit 12b
the processing programs P1 to P12 associated with the measurement
items for performing arithmetic-logic operations on the data stored
in the first memory 7 or the second memory 11. In the processing
programs P1 to P12, measurement of the above items is carried out
using the filtering, peak level calculations, calculations of the
waveform period, calculations of the phase difference of two
signals, signal extraction by a level window, signal extraction by
a periodic window and calculations of the ac and dc signal
components as described above.
The processing for measuring the above-mentioned TE signal level
and the E-F phase difference is hereinafter explained.
In measuring the TE signal level and the E-F phase difference, the
characteristics inspection device 1 performs the processing using
the rotational eccentricity of the optical disc.
As explained with reference to FIGS. 5 and 6, the rotational
eccentricity of the optical disc is induced if the center of the
optical disc set on the test bench 3, for example, is not
coincident with the axis of disc rotation. Moreover, even if the
axis of rotation is coincident with the optical disc center, the
rotational eccentricity of the optical disc is induced if the
optical disc is not set at right angle to the axis of rotation but
is set with an inclination relative to the axis of disc rotation.
In addition, the rotational eccentricity of the optical disc is
induced due to, for example, warping caused to the disc. That is,
the rotational eccentricity of the optical disc is induced if the
circumference defined by the track is deviated from the center of
the optical disc.
In measuring the TE signal level and the E-F phase difference, the
servo control circuit 5 turns on the focusing servo to effect
focusing servo control, while turning off tracking servo so as not
to effect tracking servo control. That is, with the present
characteristics inspection device, the laser light beam is
illuminated with the optical pickup 2 fixed in its position.
The arithmetic-logic unit 12d reads out digital data corresponding
to the signals E and F from the second memory 11. Then, data
corresponding to the signals E and F and the TE signal
corresponding to the signal E minus signal F are computed and
displayed for the user by the output unit 12c. FIG. 7 shows an
example of measured data displayed by the output unit 12c.
Specifically, FIGS. 7A, 7B and 7C show the signal E, signal F and
the signal TE corresponding to (E-F).
The arithmetic-logic unit 12d detects peak values of the signal E,
signal F and the signal TE read out from the second memory 11. The
arithmetic-logic unit 12d then measures the interval between the
peak values to find the periods of the respective waveforms. The
arithmetic-logic unit 12d then compares the periods thus found with
pre-set thresholds to detect signals having the waveforms exceeding
pre-set periods. The arithmetic-logic unit 12d then finds data of
the signals E, F and TE freed of data of signal portions the
periods of which exceed pre-set threshold values.
The arithmetic-logic unit 12d thus removes the portions of the
signals E, F and TE the periods of which exceed the pre-set
thresholds to derive signals freed of unstable signal portions
caused by reciprocation of the illuminated portions on the optical
disc of the laser light beam caused by eccentricity. Specifically,
the waveform signals the periods of which exceed pre-set periods
are those signals in the vicinity of the turning points of the
reciprocating movements of the illuminated positions of the laser
light beam caused by eccentricity of rotation represented by
signals from time t11 until time t12 in FIG. 3.
Based on the data freed of the signal portions the periods of which
exceed pre-set periods, the arithmetic-logic unit 12d finds the E-F
phase difference between the signals E and F while detecting an
average value of the TE signal to find the TE signal level.
With the characteristics inspection device 1, as described above,
the test bench 3 causes the optical disc to be rotated in an
eccentric state and signal portions of the detected signals E, F
and TE the periods of which exceed pre-set periods are removed to
measure the E-F phase difference and the TE signal level. Thus it
is possible with the characteristics inspection device 1 to produce
the E, F and TE signals freed of unstable signal portions produced
at the turning points of reciprocating movements of the illuminated
positions of the laser light beam on the optical disc thus enabling
measurement of the accurate E-F phase difference and TE signals
freed of noise.
Moreover, in the characteristics inspection device 1, the signals A
to F as outputs signals of the photodetectors of the optical pickup
2 are directly converted into digital data for measuring
characteristics of the optical pickup 2. Thus, stable measurement
can be realized by the characteristics inspection device 1 without
being affected by fluctuations in initial characteristics of, for
example, the servo control circuit 5 or by changes with lapse of
time. In addition, since data of the turning points are removed
after conversion to digital data, there is no necessity of
providing filters complicated in structure. On the contrary,
noise-free accurate E-F phase difference and TE signal level can be
easily measured if only it is determined whether or not the
peak-to-peak periods of, for example, the TE signal, exceeds a
pre-set threshold value.
Although the foregoing description has been made on the
characteristics inspection device 1 adapted for measuring the
characteristics of the optical pickup 2, this device 1 can also be
adapted for inspecting the characteristics of an optical disc. That
is, while an optical disc set on the test bench 3 is used as a
reference, it is also possible to measure characteristics of an
optical disc by employing an optical pickup as a reference.
Although the optical pickup 2 measured by the characteristics
inspection device 1 measures the signals A to F using the
photodetectors shown in FIG. 6, the present invention is not
limited to the use of such optical pickup. For example, the present
invention is applicable to an optical pickup for a magneto-optical
disc or an optical pickup for a phase transition disc. In these
cases, the photodetector differs in structure from the
photodetectors explained with reference to FIG. 2, so that the
numbers of the sample-and-hold circuits 8a to 8f or the second
analog-to-digital conveters 10 correspond to the number of the
photodetectors. On the other hand, if the disc is a magneto-optical
disc, the playback signal is a difference signal exploiting the
Kerr effect. Therefore, the processing contents of the program in
the arithmetic-logic unit 12d is matched to this difference
signal.
In summary, in embodiments of the characteristics measuring device
for the optical pickup and/or the optical disc, the optical disc is
rotated in an eccentric state, and the signal portions in the
vicinity of the transition point of the movement direction of the
illuminated position of the laser light beam moved by this
eccentricity are removed for measuring characteristic values of the
signal for generating the tracking error signals.
In embodiments of the method for measuring the characteristics of
the optical pickup and/or the optical disc, the optical disc is
rotated in an eccentric state, and the signal portions in the
vicinity of the transition point of the movement direction of the
laser light beam moved by the eccentricity are removed for
measuring the characteristic values of the signal for generating
the tracking error signals.
With the characteristics measuring device according to embodiments
of the present invention, the optical disc is rotated in the
eccentric state, and the signal portions in the vicinity of the
transition point of the movement direction of the illuminated
position of the laser light beam moved by this eccentricity are
removed for measuring the characteristic values of the signal used
for generating tracking error signals.
This enables characteristic values of the tracking error signals to
be measured accurately in an error-free manner with the optical
pickup and/or the optical disc.
With the characteristics measuring method according to embodiments
of the present invention, the optical disc is rotated in the
eccentric state, and the signal portions in the vicinity of the
transition point of the movement direction of the illuminated
position of the laser light beam moved by this eccentricity are
removed for measuring the characteristic values of the signal used
for generating tracking error signals.
This again enables characteristic values of the tracking error
signals to be measured accurately in an error-free manner with the
optical pickup and/or the optical disc.
The embodiments have been advanced by way of example only and
modifications are possible within the spirit and scope of the
appended claims.
* * * * *